Method of making steel cartridge cases

ABSTRACT

A cartridge case is formed from C1008 steel strip. The cartridge case is heated in a carbon rich atmosphere to increase the carbon content of the steel to approximately 0.55% and is furnace cooled so that it has a ferrite and pearlite structure and exhibits a uniform hardness and has a tensile strength similar to that of a brass cartridge case.

United States Patent [191 Bolen et a1.

1 1 METHOD OF MAKING STEEL CARTRIDGE CASES {75] Inventors: Robert J.Bolen, Port Penn, De1.;

Kelley R. Chadwick, Richfield Springs, N.Y.; Richard T. Gall, NewCastle, Del.

[73] Assignee: Remington Arms Company, Inc.,

Bridgeport, Conn.

{22] Filed: Apr. 19, 1973 [21] Appl. No.: 352,559

152] US. Cl l48/l2.1, 29/1.3, 102/43, 148/165 {51] Int. Cl C21d 9/00[581 Field of Search 148/121, 16.5; 29/13;

[56] References Cited UNlTED STATES PATENTS 2,207,289 7/1940 Fleischmann148/l6.5

2.299.138 10/1942 Grier 148/165 2.531.731 11/1950 Hibbert 148/1652,881,109 4/1959 Thern 148/l2.1

2,997,774 8/1961 Lyon 29/121 Mar. 25, 1975 3,614,816 10/1971 Weyhmulleret a1. 29/].3

OTHER PUBLICATIONS Through-Carburizing of Low Carbon Steel PermitsPurchasing and Fabricating Economies, Materials & Methods, Rose, June1948, pp. 68 71.

The Steel Cartridge Case, Norris, Ordnance, May-- June, 1954, pp.930-934.

Surface Hardened Stainless Steels, Malcolm, Product Engineering, Jan.1949, pp. 84-87.

Primary Examiner-L. Dewayne Rutledge Assistant Examiner-Arthur J.Steiner Attorney, Agent, or Firm-John H. Lewis, .lr.;Nicho1as Skovran[57] ABSTRACT A cartridge case is formed from C1008 steel strip. Thecartridge case is heated in a carbon rich atmosphere to increase thecarbon content of the steel to approximately 0.55% and is furnace cooledso that it has a ferrite and pearlite structure and exhibits a uniformhardness and has a tensile strength similar to that of a brass cartridgecase.

8 Claims, 2 Drawing Figures PATENTED 3.873.375

/ CIOO8 STRIP I BLANK 8. CUP

WASH 8 DRY CUP ANNEAL DRAW TRIM

I HEAD DEGREASE, WASH 8\ DRY i CARBURIZE 8 FURNAOECOOL i PLATE METHOD OFMAKING STEEL CARTRIDGE CASES This invention relates to the manufactureof ammunition components from steel and more particularly to a method ofmanufacturing from low carbon steel a rim fire cartridge case whichfunctions in a manner substantially the same as a brass cartridge case.

Prior to this invention, cartridge cases have generally beenmanufactured from brass because it has been the only metalsatisfactorily meeting the requirements of ammunition components inregard to cost, shaping, strength, corrosion resistance andextractibility from a gun. However, brass has many disadvantages amongwhich are the tendency toward stress corrosion cracking, the tendency toreact or cause the primers or powder contained therein to becomeunstable, and a steadily increasing cost.

For this reason, many attempts have been made to manufacture cartridgecases from other materials, one of the most popular materials havingbeen steel. Because steel does not work harden to the extent that brassdoes, prior art methods have used steel having a sufficiently highcarbon content to provide the hardness and strength required for acartridge case. However, this hard steel caused unacceptably high toolwear with the result that this manner of cartridge case manufacture wasfound to be impracticable. Low carbon steels could be used for cartridgecase manufacture without the excessive tool wear caused by high carbonsteels. However, the low strength and poor recovery characteristics ofthe resulting cartridge cases rendered them unsafe and unacceptable foruse. Thus, prior to this invention, no satisfactory method ofmanufacturing an acceptable steel cartridge case has been devised.

In accordance with this invention, readily commercially available lowcarbon steel strip is manufactured into a rim fire cartridge case whilein its relatively soft condition so that tool wear is minimized. Carboncontent of the steel is then raised to increase the tensile strength andimprove the recovery of the cartridge case. It is then cooled slowly togive the steel a ferrite and pearlite structure which exhibits strengthand ductility substantially the same as cold worked brass.

It is an object of this invention to provide a method of producing steelcartridge cases which exhibit the physical characteristics of brasscartridge cases but tend not to split during firing.

It is another object of this invention to produce steel cartridge casesby a method which provides a sufficiently hard and strong cartridge casewhile minimizing tool wear during manufacture.

These and other objects and advantages of this invention can best bedescribed with reference to the appended drawings wherein:

FIG. 1 is a sectional view of a rim fire cartridge case with anexaggerated clearance between the case and the chamber of the gun. Thebulging of the shell due to firing pressures and a split in the body ofthe case are also exaggerated.

FIG. 2 illustrates a flow sheet of the improved method of making steelcartridge cases showing the steps thereof.

The purpose of a cartridge case is to serve as a container or carrierfor the powder and the priming mixture which initiates the combustion ofthe powder. The projectile is placed in the mouth of the cartridge caseand is propelled from the barrel by expanding gas after the primer ofthe cartridge has been struck by the firing pin to ignite the propellantpowder. When in position in the gun, the shell is located in the chamberthereof; the shell must of course be properly sized during manufactureso that it will fit into the chamber. It is obvious, however, that acertain clearance will always be present between the outside of the caseand the chamber; also, there is a variation in the snugness of the fitof the case in the chamber and the relative roughness of the chambers indifferent guns. All of these factors affect the operation of thecartridge and extraction thereof.

Referring to FIG. 1, in which a conventional rim fire cartridge case isshown, a bolt 10 of a gun is shown closed against the head 11 of thecase 12, the case 12 fitting within the chamber 13 of the barrel 14. Anextractor 15 of a conventional type engages the rim of the case 12 as itis pushed into the chamber 13. A suitable striker or firing pin 16 isprovided to strike the rim of the case 12 wherein the priming mixture iscontained.

It is necessary that the material of the case have sufficient strengthand a sufficiently high yield point so that upon the combustion of thepropellant charge and under the high pressure developed thereby it willnot be forced a substantial amount beyond this yield point, because thiswould cause the sides of the shell to permanently expand, producing abulge, such as is shown in exaggerated form at 17. It is desirable,however, that at the time of firing the shell be resilient enough sothat it will spring outwardly and seal the chamber to prevent the escapeof gases rearwardly from the chamber of the barrel. In the event thatthe pressure is such as to exceed the yield point of the metal in thebody of the shell and allow too great a permanent deformation to takeplace, it is obvious that the bulging portion of the shell will tightlyengage the side of the chamber 13. The frictional force caused by thisengagement will require that an unusually high extraction force byexerted by the extractor 15. This force may be sufficiently great tocause the extractor to pull over the rim of the shell as the bolt ismoved backwardly, leaving the expended shell within the chamber andcausing great inconvenience and operational difficulty. Even if theextractor does engage and withdraw the bulged shell, an excessive forceis required to operate the gun which is, of course, undesirable.

It is seen, therefore, that the yield point of the metal bears animportant relationship to the extraction force necessary to withdraw theshell and also to the sealing of the powder gases and the prevention ofthe escape thereof through the mechanism of the gun and into the face ofthe operator. Such leakage of gas is also undesirable inasmuch as itdetracts from the power propelling the projectile through the barrel ofthe gun.

An additional requirement of the metal of the case is that its ductilitybe such that, upon the firing of the cartridge, splits, such as the oneindicated in exaggerated form at 18, will not occur. The base of theshell must also be sufficiently yielding that the blow of the firing pinwill be properly transmitted to the priming mixture within the rim andcause the same to be ignited.

Referring now to FIG. 2, M81 C1008 steel strip, that is, carbon steelstrip having a carbon content of about 0.08%, which has been coppercoated, normalized and spheroidized is used, in accordance with thisinvention, for the manufacture of cartridge cases. This material issufficiently soft so that tool wear, although greater than experiencedin the manufacture of brass cartridge cases, is minimized. It should benoted that while other low carbon steels may be used, C1008 steel isused in the preferred embodiment because of its low cost and commercialavailability.

The steel strip is blanked and cupped in a manner well known to thoseskilled in the art. The cups are then washed and dried, to remove anylubricant which may have been applied during blanking and cuppingoperations, and furnace annealed to eliminate work hardening and put thesteel in its softest possible condition for the remaining operations.

Each cup is drawn, to extend its length and provide the proper wallthickness, and trimmed, to remove the rough edge formed at its mouth.The drawn and trimmed cup is then headed whereupon it attains the finalshape and dimensions of a rim fire cartridge case; the finished steelcartridge cases are degreased, washed and dried.

As has been previously indicated, because steel does not work hardennearly as much as brass, a C1008 steel cartridge case would be unable towithstand the pressures generated during firing of a cartridgeconstructed therefrom. Accordingly, the steel must be sufficientlystrengthened after the cartridge cases have been formed so that thesteel will exhibit those physical characteristics required for safe andefficient operation.

The C1008 steel cartridge cases are, therefore, placed in an oven andheated in a carbonaceous atmosphere at a temperature in the austeniteregion, preferably about l,650F., for about ninety minutes. Thiscarburization of the cartridge cases causes carbon diffusion to occur,increasing the carbon content of the steel from 0.08% to an amountpreferably not substantially less than 0.47% nor substantially greaterthan 0.62%, or about 0.55%. At 1,650F., the carburizing temperature, thesteel is in its austenitic state. After the cartridge cases have beenfully carburized, they are moved to a cooling chamber and cooled in anoxidationpreventive atmosphere at the most rapid rate possible withoutthe formation of martensite. In the preferred embodiment, the carburizedcartridge cases are cooled in a nitrogen-hydrogen atmosphere to atemperature of about 200F. in one hour to form a ferrite and pearlitestructure which has the maximum hardness and strength obtainable withthat structure and carbon content.

The cartridge cases are then brass plated to prevent oxidation and maythen be primed and loaded by any desired method.

The method of forming small articles, such as intricate stampings, fromlow carbon steel and subsequently carburizing the objects to increasetheir carbon content and, accordingly, their hardness is known. However,such a process has not heretofore been useable in the manufacture ofsteel cartridge cases.

The teaching of the prior art is to heat the objects in a carbonaceousatmosphere and then quench and temper them to further enhance theirhardness and strength. Although this process is ideal for the productionof decorative steel Objects, it does not produce acceptable cartridgecases.

It has been shown herein that the steel cartridge cases are notcarburized until after all forming steps have been completed so that theexcessive tool wear which would occur from working high carbon steel isavoided. It is well known that when carbon steel is austenitized andthen quenched, some distortion of the quenched object results from thetransformation of austenite to martensite. Although this distortion maynot be readily noticeable in small decorative objects, it cannot betolerated in the manufacture of cartridge cases wherein dimensionaltolerances are very small.

Additionally, cooling the steel by quenching produces a martensiticstructure which inherently has a low ductility. When subjected to thepressures produced when a cartridge is fired, a steel cartridge casewith such a martensitic structure would have an even greater tendency tosplit than a brass cartridge case. Of course, the ductility of themartensite could be increased by tempering for extended periods of time.However, such tempering would also decrease the strength of thematerial.

The hardness of the quenched and tempered martensitic structure causesanother problem in that it produces excessive wear of gun parts, such asthe firing pin and extractor, and reduces the sensitivity of the rim ofthe cartridge case so that misfires may occur due to the resultinginability of the firing pin to apply sufficient compressive forcethrough the case to explode the primer. For these reasons, thecarburization process, as disclosed in the prior art, has never beenadaptable for the production of steel cartridge cases.

Steel cartridge cases made in accordance with this invention avoid theproblems heretofore encountered with steel cartridge cases. In fact,these cartridge cases exhibit physical properties very similar to thoseof brass cartridge cases.

In the method of this invention, the carburized steel cartridge casesare furnace cooled to have a ferrite and pearlite structure. Because ofthis slower cooling procedure, the distortion which occurs whenaustenite is quenched to produce a martensitic structure is preventedand the cartridge cases have the same dimensions after carburization asthey did when they were formed.

Unlike martensite, the ferrite and pearlite structure has very highductility. Steel cartridge cases made in accordance with this inventionexhibit an extremely low incidence of splitting. During comparisonfiring tests of 22 caliber rim fire cartridges in twelve differentrifles, no splits were recorded for steel cartridge cases made inaccordance with this invention whereas splits were observed in 0.17% ofthe brass cartridge cases and 0.46% of the quenched and tempered steelcartridge cases tested. During the entire test, in which 4,400cartridges, having steel cartridge cases made in accordance with thisinvention, were fired in both rifles and pistols, only one cartridgecase showed any evidence of splitting. In a later test, 6,200 suchcartridges were fired in rifles and pistols without the occurrence of asingle cartridge case split.

Steel cartridge cases made in accordance with this invention have beenfound to produce only slightly more wear to firing pins and extractorsthan do brass cartridge cases and significantly less wear than doquenched and tempered steel cartridge cases. The ferrite and pearlitesteel cartridge cases have a hardness range of about -200 KHN which issubstantially the same as the hardness range exhibited by brasscartridge cases. Because the steel cartridge cases are heat treatedduring the carburization process, after the completion of all coldworking, the hardness tends to be more uniform over the length of thesecases than the hardness of brass cartridge cases which are used in theircold worked state. The average tensile strength of the ferrite andpearlite steel cartridge case of this invention is about 90,000 psi,substantially identical to the average tensile strength of a brasscartridge case. It has been found that the tensile strength exhibited bythe steel cartridge cases was generally constant in value throughout the0.47% to 0.62% range of carbon content.

It should be apparent from the foregoing description that a new methodhas been disclosed for manufacturing a novel steel cartridge casestructure which incorporates the advantages of brass cartridge caseswithout their inherent splitting tendency and at a lower cost.

We claim:

1. A method of manufacturing a cartridge case from low carbon steelcomprising the steps of forming the steel into the shape of a cartridgecase, carburizing the cartridge case to increase the carbon content ofthe steel, and cooling the cartridge case in a gaseous medium to givethe steel a ferrite and pearlite structure.

2. The method of claim 1 wherein said cartridge case is carburized to acarbon content not substantially less than 0.47% and not substantiallygreater than 0.62%.

3. The method of claim 1 wherein said cartridge case is carburized to acarbon content of approximately 0.5 5%.

6 4. The method of claim 1 wherein said cartridge case is carburized ata temperature of approximately 1,650F.

5. The method of claim 4 wherein the cooling of said cartridge case isto a temperature of about 200F. in

heating the cartridge case in a carbonaceous atmosphere at a temperatureof about l,650F. to increase the carbon content of the steel to a valuenot substantially less than 0.47% and not substantially greater than0.62%, and cooling the cartridge case to a temperature of about 200F. in1 hour.

8. The method of claim 7 wherein the carbon content of the steel isincreased to approximately 0.55%.

1. A METHOD OF MANUFACTURING A CARTRIDGE CASE FROM LOW CARBON STEELCOMPRISING THE STEPS OF FORMING THE STEEL INTO THE SHAPE OF A CARTRIDGECASE, CARBURIZING THE CARTRIDGE CASE TO INCREASE THE CARBON CONTENT OFTHE STEEL, AND COOLING THE CARTRIDGE CASEIN A GASEOUS MEDIUM TO GIVE THESTEEL A FERRITE AND PEARLITE STRUCTURE.
 2. The method of claim 1 whereinsaid cartridge case is carburized to a carbon content not substantiallyless than 0.47% and not substantially greater than 0.62%.
 3. The methodof claim 1 wherein said cartridge case is carburized to a carbon contentof approximately 0.55%.
 4. The method of claim 1 wherein said cartridgecase is carburized at a temperature of approximately 1,650*F.
 5. Themethod of claim 4 wherein the cooling of said cartridge case is to atemperature of about 200*F. in one hour.
 6. The method of claim 1wherein said cartridge case is cooled in a gaseous medium comprising amixture of nitrogen and hydrogen.
 7. A method of manufacturing acartridge case from steel strip having a carbon content of about 0.08%,said method comprising the steps of cutting a blank from the steelstrip, forming the blank into tubular member closed at one end, headingthe tubular member to form a cartridge case with a peripheral rim at theclosed end, heating the cartridge case in a carbonaceous atmosphere at atemperature of about 1,650*F. to increase the carbon content of thesteel to a value not substantially less than 0.47% and not substantiallygreater than 0.62%, and cooling the cartridge case to a temperature ofabout 200*F. in 1 hour.
 8. The method of claim 7 wherein the carboncontent of the steel is increased to approximately 0.55%.